Integrated circuit (IC) chips are used and incorporated in virtually every segment of modern electronic and computer products. For example, modem products such as computers, telephones, electronic goods, etc. typically include one or more IC chips. As is well known in the art, the IC chips often include switched capacitor circuits, which are used to realize many analog and mixed-signal circuits including filters, data converters, communication circuits, etc.
One of the main elements of a switched capacitor circuit is a switch. In an IC chip setting, complementary metal-oxide semiconductor (CMOS) switches are often used to take advantage of their high speed, small sizes, and zero turn-on voltage drop. Prior Art FIG. 1 illustrates a conventional CMOS switch 100 that can be used in an IC chip. The conventional CMOS switch 100 includes a n-channel MOS (NMOS) transistor M1 and a p-channel MOS (PMOS) transistor M2. The NMOS transistor M1 and the PMOS transistor M2 are coupled to each other in parallel at the common source node 102 and at the common drain node 104. When activated, the CMOS switch 100 receives an input signal Vin at the node 102 and transmits the Vin as an output signal Vout at the node 104.
In order to activate the CMOS switch 100, the gate of the NMOS transistor M1 is coupled to a supply voltage rail Vdd and the gate of the PMOS transistor M2 is coupled to the ground potential. The body (e.g., substrate or bulk) of the transistor M1 is tied to the ground potential while the body of the transistor M2 is tied to the supply voltage Vdd. The transistors M1 and M2 are coupled to each other at the source and drain nodes 102 and 104.
When designed with standard transistor components, the CMOS switch 100 properly functions with a supply voltage Vdd of 5 V or greater. However, today's IC chips increasingly employ lower supply voltages (e.g., 3 V) than 5 V as the feature sizes continuously shrink in an IC chip. The use of the lower power supply saves power and thus is advantageous in many applications including mobile computing and communication fields.
Unfortunately, the use of a lower supply voltage such as 3 V in the CMOS switch setting presents a persistent body effect (e.g., back gate bias effect) that may adversely affect the switching of the transistors M1 and M2. Specifically, at a low supply voltage, the transistors M1 and M2 in the CMOS switch 100 may not turn on properly due to the lower gate overdrive combined with the body effect in the transistors M1 and M2. Body effect occurs in an MOS transistor when the body is at a different potential than the source/drain such that a reverse biased junction is formed between the source/drain and the body (i.e., substrate) of the transistor. The reversed biased PN junctions cause a depletion region to form around the associated drain or the source.
For example, as the substrate or body (e.g., p-type silicon) of an NMOS transistor is made more negative relative to the source or drain (e.g., n-type silicon) of the transistor, the depletion region between the substrate and the sourcc/drain experiences a larger potential drop and thus becomes thicker. Accordingly, in order to turn on the transistor, a higher voltage must be applied at the gate of the NMOS transistor to overcome the larger depletion region. The net result of the body effect is the apparent increase in the effective threshold voltage V.sub.TH,NOMOS of the NMOS transistor as the reverse bias between the substrate and the source or drain is increased. Similarly, the effective threshold voltage .vertline.V.sub.TH,PMOS .vertline. of a PMOS transistor will increase if its body (e.g., n-type silicon) is at a higher potential than its source or drain (e.g., p-type silicon).
By way of example, the transistors M1 and M2 in CMOS switch 100 without body effect are characterized by a threshold voltage V.sub.TH =V.sub.TH,NMOS =.vertline.V.sub.TH,PMOS .vertline.=0.8 volts. If the body effect is assumed to add 0.5 volt, the apparent threshold voltage VTH of the transistors M1 and M2 is 1.3 volt (0.8+0.5 volt). With a 3 V supply voltage Vdd and the voltages at the input and output nodes at about 1.5 volts (i.e., Vdd/2), the margin between the apparent threshold voltage and the gate-to-source voltage Vgs (e.g., gate overdrive) is only 0.2 volt. With such a narrow margin, the CMOS switch 100 will not operate reliably. Furthermore, the supply voltage of the 3 V rated supply voltage Vdd may actually fluctuate between 2.7 V to 3.3 V. When the supply voltage is at 2.7 V, the voltage margin decreases even more. Accordingly, the transistors M1 and M2 may not turn on properly.
One obvious solution to the switching problem caused by the body effect is to use a higher supply voltage. For example, higher supply voltages such as 5 V overcome the body effect by applying a high gate voltage that compensates for the body effect. Another technique uses a special clock booster or charge pump circuit to provide a larger gate overdrive to the affected transistors. Unfortunately, such circuits create internal voltage much higher than the supply voltage. Consequently, these circuits typically require special high-voltage transistor structures. However, the standard sub-micron commercial silicon processes usually do not implement such structures due to higher fabrication cost. Additionally, the small transistor size in such processes means that the regular transistors will have relatively low breakdown voltage.
Another solution may employ low-threshold transistors having threshold voltages of, for example, 0.3 to 0.4 volt, to overcome the body effect. This solution, however, is more costly since low-threshold voltage transistors require extra process steps. However, most commercial CMOS processes typically do not have this option. Moreover, the low-threshold voltage transistors have significant current leakage through them even when they are turned off.
Thus, what is needed is a circuit and method for switching a CMOS switch at low supply voltages without requiring costly and complex circuit structures. What is also needed is a circuit and method that eliminates the body effect in a transistor used in a CMOS switch.